Insights into the Electrochemical Hydrogenation Reaction Mechanisms of Aromatic Compounds Mixtures over Bimetallic PtRu Catalyst

doi:10.21203/rs.3.rs-4901664/v1

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Insights into the Electrochemical Hydrogenation Reaction Mechanisms of Aromatic Compounds Mixtures over Bimetallic PtRu Catalyst

https://doi.org/10.21203/rs.3.rs-4901664/v1

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Efficient and selective electrochemical hydrogenation (ECH) of organic compounds is crucial for global sustainability; offering both chemical feedstock circularity and routes to generate synthetic fuels, reducing fossil fuel reliance. This study focuses on ECH of benzoic acid (BA), phenol (P), and guaiacol (G), commonly appearing as mixtures in potential oils for upgrade. The impact of various conditions on the hydrogenation process was elucidated using a carbon-supported PtRu catalyst; assessing initial concentrations/mixtures, temperatures, and current densities. Phenol ECH exhibited the most favourable outcome, achieving an 83.17% conversion rate and 60% Faradaic Efficiency (FE). When mixtures (BA + P, BA + G, P + G, and BA + P + G) were evaluated, revealing that BA + P yielded the highest conversion rate (64.19%) and FE (74%). This outcome suggests a potential synergistic effect between benzoic acid and phenol. Additionally, regardless of experimental parameters, BA selectivity for cyclohexane carboxylic acid (CCA) remained consistently at 100%. Density functional theory (DFT) calculations provided insight into molecular/catalyst interactions, highlighting the stability of parallel positioning the BA molecule on the cathode surface compared to perpendicular placement (-1.12 eV vs. 0.58 eV), this offers a plausible explanation for the observed high selectivity towards CCA. These findings provide a foundation for future developments in ECH of real pyrolysis oil.

Physical sciences/Chemistry/Electrochemistry/Electrocatalysis

Physical sciences/Chemistry/Green chemistry/Biofuels

Pyrolysis is the thermal decomposition of materials at high temperatures in an inert atmosphere. When applied to waste and biomass feedstocks, the condensable liquid product from pyrolysis (constitutes around 70 wt.% of the pyrolysis products), known as bio-oil is widely recognised as a low-carbon biofuel and precursor for chemical production. In waste industry, there is significant interest in applying pyrolysis as an enabling technology for plastic polymer recycling processes, producing pristine polymers as platform chemicals. This can potentially address the global issue of low levels of plastic recycling; currently standing at less than 10% in the European Union, most of which uses mechanical recycling routes (i.e. limited to thermoplastics). ^1–3

Despite these potentials, bio-oil, is a complex mixture containing over 300 compounds which pose significant challenges for its practical use⁴. Its physical and chemical properties, including viscosity, C:H C:O ratios, and thermal and chemical stability, typically cannot meet the standards direct application as a biofuel or as feedstock for the chemical and petrochemical industries⁵. Thus, upgrading bio-oil is essential to enhance its suitability for widespread use.

A key step to improving bio-oil quality for fuel and polymer recycling applications is to increase the hydrogen-to-carbon ratio crucial for stability through hydrogenation processes. Conventional hydrogenation technologies, including catalytic cracking and hydroprocessing. Both processes involve high temperature and pressure reactions. To date, commercial applications of these hydrogenation methods are rare due to the significant operational costs, catalyst deactivation and undesirable secondary reactions, particularly tar formation ^6–10. Electrochemical hydrogenation (ECH), by contrast, can be performed under milder reaction conditions at low temperatures (below 80°C) and ambient pressure to reduce reactive functionalities. Additionally, ECH process requires no H₂ supply to the process, therefore eliminates the costs and risks associated with hydrogen storage. These advantages make ECH a promising alternative to conventional routes for sustainable bio-oil upgrading ^11–13.

However, as a proof-of-concept currently, fundamental reaction kinetics and mechanisms of key bio-oil constituent compounds during ECH process are still not fully understood. As such, this study uses key compounds, including guaiacol, phenol, benzoic acid and their derivatives commonly found in both biomass and plastic-derived bio-oil^14–16 as model compound, to investigate there reaction mechanisms in a H-type cell set up.

H-type cell is the most common design for electrochemical reactions. It consists of a anode and a cathode chamber, separated by a porous membrane (Fig. 1). Cathode material selection is currently one of the most important steps in ECH research and deployment, as it can influence the conversion rate, Faradaic efficiency (FE), and selectivity. For phenol and guaiacol ECH, Pt, Pt/ACC (activated carbon cloth), Ru/ACC and Raney nickel have been widely reported to be effective for the ECH reactions of these compounds ^13,17–20. However, the effective ECH of benzoic acid remains a challenge due to increasing some undesired side reactions (carboxyhydrogenation and over-hydrogenation) alongside the hydrogenation of the aromatic ring²¹. A ‘bimetal’ Pt/Ru catalytic system has been reported by Du et al²¹ and Fukazawa et al²² for benzoic acid ECH, which performed well in promoting both hydrogenation and direct hydrogenolysis. Salakhum et al²³ found that combining both metals facilitates the cleavage of C-O and O-H bonds, reducing their heterolytic activation energies, which increases ECH effectiveness.

Understanding the complex interplay of model compounds during electrocatalytic hydrogenation (ECH) is crucial for translating this process into industrial applications when dealing with real bio-oil feedstock. The presence of multiple components can significantly influence hydrogenation pathways, leading to synergistic and antagonistic effects. For instance, competition for adsorption sites can alter product distribution. Consequently, an in-depth understanding of co-hydrogenation phenomena is essential to optimise ECH processes for industrial relevance.

This study investigates the mild hydrogenation of bio-oil model compounds using a PtRu/ACC catalyst. To elucidate the complex reaction pathways and kinetics during in mixtures, we adopted a full factorial experimental design to examine the co-hydrogenation of benzoic acid (BA), phenol (P) and guaiacol (G). Additionally, the ECH mechanisms of model compounds was analysed, including side reactions, adsorption energy, and molecule placement, using in situ electrochemical measurements and density functional theory (DFT) calculations.

Electrochemical hydronation of model compounds

Benzoic acid electrochemical hydrogenation tests were carried out using a H-type electrochemical cell (Fig. 1). At an initial BA concentration of 10 mM, 94.70% conversion with 100% selectivity to cyclohexane carboxylic acid (CCA) was achieved within 4 hours at 9% Faradaic efficiency (Fig. 2a-b). This high selectivity was consistently observed across all benzoic acid experiments in this study. Increasing the initial concentration from 10 to 20 mM resulted in a decreased conversion rate and increased FE (75.4% and 14%, respectively). The initial BA concentration significantly impacted both conversion rate and Faradaic efficiency (FE). Increasing the concentration from 10 to 20 mM enhanced both parameters, likely due to increased collision frequency between H⁺ ions and organic molecules. However, further concentration increases to 30 and 50 mM led to decreased conversion rates, despite slightly improved FEs. This suggests a mass transport limitation at higher concentrations, where the availability of electrons becomes insufficient to sustain the reaction rate. Based on this finding, the optimal initial BA concentration for this study was determined to be 20 mM and applied to other experiments across this study.

Temperature significantly influenced ECH performance. To evaluate the effect of temperature on the process, and determine the optimal condition, we tested temperatures of 40, 50, 55, 60 and 80°C (Fig. 2c-d), maintaining the same current density at 100.0 mA cm^− 2 and setting the initial concentration of BA to 20 mM. At lower temperatures (40°C), the system's conductivity was reduced, leading to insufficient activation energy for bond cleavage ^11,13,24 and consequently lower conversion (49.8%) and Faradaic efficiency (9%). Increasing the temperature to 50–60°C enhanced both conversion and FE, with optimal performance observed at 55°C. However, further heating to 80°C promoted hydrogen desorption from the cathode, resulting in decreased ECH activity (78.74% conversion, 14% FE).

The influence of current density on ECH was investigated using five current values (0.1, 0.2, 0.4, and 0.6 A), corresponding to current densities of 12.5, 25.0, 50.0, 100.0, and 150.0 mA cm⁻², respectively (based on a 4.0 cm² cathode area). The initial BA concentration was maintained at 20 mM, and the temperature at 55°C. Preliminary tests at 0.05 A indicated insufficient proton generation on the cathode surface, limiting reactant collisions and hindering process efficiency.

Increasing the current density from 12.5 to 25.0 mA cm^− 2 significantly improved both conversion rates and Faradaic efficiency. At a current density of 50.0 mA cm^− 2, the conversion rate increased by 13.55%, but the FE reduced significantly from 53–31%. At higher current densities of 100 and 150 mA cm^− 2, higher overpotentials are required (0.340 and 0.416 V vs Ag/AgCl, respectively), increasing HER rate and resulted in the lowest FE values observed in the experiment, at 14% and 10%, respectively. This decline in Faradaic efficiency at higher overpotentials can be attributed to several factors. First, higher overpotentials increase the likelihood of side reactions, which in this process is hydrogen evolution reaction (HER), these compete with the desired reaction and consume part of the applied current without contributing to the target product formation, reducing FE. Additionally, the rapid generation of hydrogen gas at elevated current densities can lead to the accumulation of gas bubbles on the cathode surface, which impedes effective mass transport by blocking active sites, thereby reducing the available surface area for the reaction.¹¹

To assess cathode stability, reproducibility, and process sustainability, the optimized conditions (temperature = 55°C, current = 0.1 A, initial BA concentration = 20 mM) were replicated over five consecutive cycles. The results (Fig. 3) indicated a consistent performance of the PtRu/ACC catalyst over the five cycles of the experiment. Despite the aggressive reaction environment, there is no sign of cathode degradation, which was confirmed by scanning electron microscopy observation (Fig. 9) in addition to assessing its consistency in performance via GC-MS. It is worth noting that the selectivity for CCA remains 100% throughout all cycles.

Phenol and guaiacol were subjected to ECH under the optimized conditions determined for benzoic acid. Phenol was observed to be the most effective under these conditions, achieving 83.17% conversion and 60% Faradaic efficiency with high selectivity (99%) towards cyclohexanol (Fig. 4a-b). In contrast, guaiacol demonstrated lower activity (68.59% conversion, 59% FE) (Fig. 4c-d) with a product distribution of 50% cyclohexanol, 46% 2-methoxycyclohexanol, and 4% 1-methoxycyclohexane, indicating a preference for demethoxylation over dehydration. Notably, complete aromatic ring hydrogenation was observed for all three model compounds, suggesting a common reaction pathway.

While higher conversion rates and FE for benzoic acid ECH have been reported in literature using smaller H-type cells (25 mL vs. 90 mL) ²¹, direct comparison is challenging due to differences in cell geometry. Smaller cells often exhibit lower resistance and require lower overpotentials, which in turn affects FE. Scaling up to larger H-type cells necessitates increased reactant quantities, higher currents, and overpotentials, potentially impacting efficiency and selectivity. To address these limitations, continuous flow reactors may offer advantages by mitigating mass transfer issues and enabling better process control.

Electrochemical hydrogenation of model compound mixtures

Understanding the effects of having more than one model compound during ECH is essential to progress bio-oil upgrading, which are inherently mixed compounds. To investigate potential interactions (synergistic and antagonistic effects) of each compound in the mixture, a factorial experimental design approach was applied as shown in Table 1, where a “+” signs indicate the compound is present, and a “-” signals the opposite. Notably, the applied overpotential showed minimal variance (0.003V), remaining between 0.205 and 0.208 V vs Ag/AgCl, regardless of the composition or concentration of the model compounds. This indicates that the conductivity of the system is more significantly influenced by the electrolyte and temperature rather than the model compounds, likely due to their low concentration.

Table 1

Results comparison for the ECH of benzoic acid (BA), phenol (P) and guaiacol (G) mixtures. Reaction parameters: current density = 25.0 mA cm^-2, temperature = 55°C, electrolyte = H₂SO₄ 1M.
Compound			Entry	Initial concentration (mM)	Conversion (%)	FE (%)
BA	P	G	Entry	Initial concentration (mM)
+ - - + + - + + + - +	- + - + - + + + - + +	- - + - + + + - + + +	(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11)	20 20 20 40 40 40 60 20 20 20 20	77.35 88.75 68.59 64.19 57.27 47.80 42.59 80.02 63.01 74.78 33.64	56 64 59 74 66 55 73 58 46 54 24

It was observed that increasing the number of unsaturated molecules generally decreased conversion rates, with the exception of mixtures containing all three compounds (Entries 7 and 11). Conversely, FE increased in runs 4, 5 and 7. Figure 2a shows a clear correlation between decreasing conversion rates and increasing initial concentration.

Notably, in entry 4 a significant drop in conversion from 83.05% (average of BA and P) to 64.19% in the mixture was observed, accompanied by a 13% increase in FE. This suggests a possible synergistic interaction between these two compounds, which aligns with previous studies on phenol-containing mixtures (phenol + furfural ²⁵, and phenol + benzaldehyde^26,27), where hydrogen-bonded complex formation enhanced the hydrogenation of the co-reactant.

Phenol exhibited the highest conversion rates among the model compounds. In the BA + P mixture (Entry 4), phenol conversion reached 88.57% compared to 39.81% for benzoic acid. Similarly, phenol was preferentially converted in the G + P mixture (Entry 6) at 74.16% compared to 21.43% for guaiacol. This trend persisted in the ternary mixture (Entry 7), with phenol conversion at 75.79% exceeding those of benzoic acid (18.94%) and guaiacol (33.04%). Phenol also demonstrated the highest overall conversion efficiency as a standalone compound.

As a standalone compound, guaiacol consistently exhibited lower conversion rates compared to phenol and benzoic acid. This reduced reactivity is attributed to its complex molecular structure containing both methoxy and phenolic groups, resulting in higher bond energies²⁰. Interestingly, in mixtures, the hydrogenation of guaiacol appears to be favoured when compared to BA. Mixing benzoic acid with guaiacol (entry 5), led to 50% and 65% conversion of each compound, respectively. This value is quite similar to pure guaiacol tests (69%, entry 3), despite the higher initial concentration. This trend was also observed for entry 7 (mixture of all three compounds). All these results are backed by the LSV (linear sweep voltammetry) analysis (see Fig. 8), where the hydrogenation of phenol is favoured over guaiacol, which is, in turn, favoured over benzoic acid.

The ECH of mixtures also led to different products, including, although in small quantities (< 2.0 area %), the full hydro-dehydroxylation of single aromatic compounds into cyclohexane. Guaiacol was also converted into phenol and cyclohexanone when mixed with BA, which complicates the calculations for the conversion rates of phenol. Where, if this behaviour is duplicated in other mixtures, it becomes increasingly difficult to precisely calculate to which extent the present molecules are the ones added into the system as a model compound or produced via ECH. Therefore, the conversion and FE of said processes could be higher than the reported values. The proposed hydrogenation path for guaiacol and phenol in mixtures is shown in Fig. 5.

Tests were carried out to observe the impact of the initial concentration on mixtures. These followed the same ratio between compounds, but the initial total concentration of compounds for ECH was set to 20 mM. For entries with two compounds, each compound contributed 10 mM, and for entries with three compounds, each compound contributed 6.7 mM.

Setting the initial concentration to 20 mM led to similar trends to the ones observed previously. The BA + P mixture (entry 8) remains the best result for a mixture, presenting a considerably high 69.70% conversion for BA and 90.34% for phenol, very similar results to entries 1 and 2, despite the reduction in the initial concentration for each compound (from 20 to 10 mM), once again remarking to possible synergistic effects between these two compounds. Interestingly, although a slight decrease in activity is to be expected, this did not occur for phenol. Phenol also remained the most active species for ECH, achieving 91.44% conversion, compared to 58.12% for guaiacol (entry 10) and 61.60% compared to 24.33% and 26.11% for BA and guaiacol (entry 11), respectively.

Although the mixtures with guaiacol still presented inferior results, it is worth noting that P + G at 20 mM (entry 10) is far superior to P + G at 40 mM (entry 6), possibly due to guaiacol’s resistance to ECH. That is, increasing guaiacol’s concentration meant a greater negative interference in the process than phenol’s positive one, leading to a lesser result. No phenol was detected in entry 9 (BA + G), and there was also no cyclohexane found in any of the experiments. This suggests that the concentration of these compounds may have been too low to detect, or that a larger quantity of reactant was needed to achieve the full conversion.

Factorial design analysis

Two models were developed to study the effects of mixture composition at low and high concentrations. In both cases, the initial model included main effects and interaction terms: BA, P, G, BA + P, BA + G, and P + G, with conversion and FE as responses.

For conversion at higher concentrations (Fig. 6a), significant contributions were observed from BA, BA × P, BA × G, and P × G, while P and G were excluded from the final model. The r² value for conversion was 0.7267, indicating that the model explains a moderate proportion of the variability in the response. For FE (Fig. 6b), all terms significantly contributed, with an r² value of 0.9290, indicating a strong model fit.

At lower concentrations, G was excluded from the final model for conversion, while BA and G were removed for FE. The r² value for conversion was 0.9608 (Fig. 6d), and for FE, r² was 0.9325 (Fig. 6e), both demonstrating a high correlation with the predicted model.

The conversion profiler (Figs. 6c and 6f) shows that increasing the concentration of the mixture generally leads to higher FE, with one exception. Phenol had the greatest positive impact on both conversion and FE, regardless of concentration. Conversely, Guaiacol decreased both responses in 3 out of 4 cases but led to a slight increase in FE at higher concentrations. These findings reaffirm the observed effects of these two compounds in the ECH process.

Residual plots for both models' responses showed that the residuals appeared random, with no discernible patterns or trends, confirming the model's assumptions. Additionally, no residuals exceeded control limits, indicating a good model fit. The plots of actual versus predicted responses showed a high correlation, confirming the model's predictive capability.

Density Functional Theory (DFT) calculations

DFT calculations were used to explain further the behaviour of benzoic acid on the cathode surface. The material was built with a Pt-Ru atomic ratio of 1:1 herein denoted PtRu(111). BA was placed both with a parallel placement and at a 45° angle into the surface and the structure was relaxed to find the adsorption energy. Figure 7 shows the interaction between them.

The lower energy value calculated for the parallel placement gives a good interpretation of the 100% selectivity of BA into CCA, whereas there is no bond between the carboxylic acid part of the molecule and the surface. The atom's placement shows that this group moves in the “z” direction, bending the molecule and moving away from the surface. That is, there is the formation of π-σ interactions between the aromatic ring and the metal surface, rather than the carboxylic group. There was also a difference in E_ad depending on the initial position of the compound, where placing the aromatic ring closer to ruthenium atoms (Fig. 7b) led to a more stable system (E_ad2 = -1.12 eV) than closer to platinum atoms (E_ad3 = -0.17 eV, Fig. 7c), signifying it as the preferential site.

Interestingly, Fig. 7b shows that a defect is created in the first layer of the cathode’s surface. To accommodate the BA molecule, the two ruthenium atoms which bonded with C1, C3, C4 and C6 moved too far from each other, severing the bond. To determine whether this was a one-time event, the atoms were placed closer to each other until a bond was formed, but the relaxation process would always lead to this defect. It is worth mentioning that from the second layer onwards, this did not occur.

It is widely acknowledged that the Langmuir-Hinshelwood (L-H) adsorption mechanism is followed by organic compounds containing a benzene ring during ECH.²¹ The process involves H⁺ being quickly adsorbed into the surface, due to the synergistic effects between Pt and Ru where, in combination with an e⁻, produces H_ads (Vomer step). The next step is the adsorption of BA in a position parallel to the catalyst surface, to form BA_ads. Subsequently, H_ads is transferred into BA_ads until the full hydrogenation of the aromatic ring is accomplished, followed by the desorption of the product (cyclohexane carboxylic acid). The proposed mechanism for the ECH of benzoic acid over PtRu/ACC is shown in Fig. 8.

The hydrogenation happens in a specific order, following the most stable state of the molecule. Calculations on the one-by-one hydrogenation (Supplementary Fig. 1) of BA show that the first carbon to receive a H⁺ is C1 (meta), followed by the order C2-C3-C4-C5-C6. Due to the carboxylic acid`s electronegativity, a negatively charged dipole (δ⁻) is formed which is close to C6, giving a good justification for why it is the least stable position for the first hydrogen (H1), and why it is the last to be hydrogenated. The positive dipole on C6 (ortho), which borders C1 and C5 (meta) and the distance from C3 to the -COOH group set them as the major candidates for H1, at a relatively small 0.056 and 0.0147 eV difference from C1 to C5 and C3, respectively. The same trend follows for the rest of the process, where the neighbouring carbon to the one that receives the hydrogen is always the least stable position, until the ring is fully stabilised.

Material characterisation

Figure 9 shows the Scanning Electron Microscopy (SEM) secondary electron image for PtRu/ACC before use and after 5 cycles of the optimised BA electrochemical hydrogenation process. In both samples, the metal deposition is homogeneous throughout the surface of the carbon cloth. Elemental mapping (by energy dispersive X-ray spectroscopy) was carried out and revealed an even distribution of both Pt (in teal) and Ru (in green) across the surface; this even distribution is beneficial to optimise the reaction. No degradation of the cathode is apparent after 5 reruns, the concentration and distribution of metals remain unaltered.

LSV analysis shows that PtRu/ACC exhibits − 0.203 V vs Ag/AgCl at 10 mA cm^− 2, while it is less negative when adding the compound, reaching − 0.176 V, -0.159V and − 0.149 V vs Ag/AgCl for BA, G and P, respectively (Fig. 10). This measurement is important for the well-known activity for hydrogen evolution reaction (HER) at this current density. The voltage change (∆E) of ∆E₁ = 0.027V, ∆E₂ = 0.044V and ∆E₃ = 0.054V shows that the ECH of all the model compounds is prior to HER, the competing reaction. When adding P to BA, the potential went from − 0.176V to -0.160V vs Ag/AgCl, a difference of 0.16V, whereas when adding G to BA, this difference was 0.08V. Adding P to G increased the potential from − 0.159 to -0.156V, a 0.03V change (see Figure S2-4).

In conclusion, the electrochemical hydrogenation of mixtures represents a significant advancement in the field of bio-oil upgrading. Through a factorial experimental design approach, we explored the interactions among model compounds, shedding light on both synergistic and antagonistic effects within the system. Our findings underscored the robustness of the electrochemical process, with the applied overpotential exhibiting minimal variance across different compositions and concentrations of model compounds. Notably, we observed a clear trend where increasing the number of unsaturated molecules in the system led to reduced conversion rates, except when all three compounds were mixed, suggesting complex dynamics at play. Phenol emerged as the most readily converted compound in most scenarios, highlighting its preferential hydrogenation over benzoic acid and guaiacol. The resistance to guaiacol hydrogenation, attributed to its complex molecular structure with methoxy and phenolic groups, posed challenges in achieving high conversion rates, particularly when guaiacol was present in mixtures. However, the hydrogenation of guaiacol was favoured over benzoic acid, indicating nuanced reactivity patterns influenced by compound interactions.

Furthermore, our investigation into the impact of initial concentration revealed consistent trends, with mixtures maintaining similar conversion patterns despite variations in compound concentrations. Phenol consistently demonstrated higher activity compared to benzoic acid and guaiacol, even at reduced concentrations, underscoring its significance in the electrochemical hydrogenation process.

DFT calculations demonstrated that positioning the BA molecule parallel (0°) to the surface was notably more stable compared to a 45° angle placement. This preference suggests a stronger affinity for the adsorption of the aromatic ring over the carboxylic acid component of the molecule. Such findings provide insight into the observed 100% selectivity of product formation towards cyclohexane carboxylic acid.

Overall, our study provides valuable insights into the electrochemical hydrogenation of mixtures, offering a comprehensive understanding of compound interactions and their influence on conversion rates and selectivity. These findings lay the groundwork for further optimisation of electrochemical processes in bio-oil upgrading, with potential applications in sustainable energy production and chemical synthesis.

Materials

Sulphuric acid solution (5M), benzoic acid (99%, extra pure) and guaiacol (99%) were purchased from Thermo Fisher Scientific. Phenol (liquified ≥ 89%) was purchased from Sigma Aldrich. All chemicals were used as received without further purification. A Nafion 117 membrane (from Ion Power) was used as the cation exchange membrane to separate the cathode and anode chambers. To ensure the activation of the membrane and optimise its ion transport capability, it was pretreated according to Peng et al ²⁸. It was treated in oxygen peroxide (H₂O₂) at 80°C for 1h, followed by deionised water for 1h and 1.0 M H₂SO₄ for 1h at the same temperature. The membrane was washed with deionised water after each stage.

ECH of pure and mixed model compounds

A two-chamber electrochemical cell, separated by a proton exchange membrane (PEM) was selected for this process, where the anode was a 1x1 cm platinum plate, the selected catalytic cathode was 4 mg/cm² Platinum Ruthenium Black-Carbon Cloth Electrode (PtRu/ACC). Both cells were filled with 1.0 M sulfuric acid (H₂SO₄) as the electrolyte. The ECH tests were performed at a 90mL (per chamber) H-type cell and run at electrostatic control and constant temperature (both at different values depending on the experiment) for 10 min without the organic compound to polarise the electrodes. After, the model compound was added to the system to obtain a set final concentration. The experiment was run for a duration which ensured the availability of sufficient electrons to convert the compounds (4 to 6 hours). Samples (1mL) were taken in discrete time intervals, and the organic compounds were extracted with 2mL of dichloromethane (DCM). After each experiment, the electrolyte was discarded, the cell cleaned and the cathode was soaked in 5ML dichloromethane (DCM) for 15min, to ensure the desorption of any molecules on its surface.

The process of finding the optimised conditions was done utilising benzoic acid as the main model compound, and guaiacol and phenol were used to confirm the effectivity of the process in other model compounds. All tests with mixtures were carried out at the same temperature and current density as the optimised point, variations on the initial concentration were specified in the text.

Table 2

Benzoic acid ECH optimisation tests and ECH of both phenol and guaiacol at the optimised conditions
Compound	Concentration (mM)	T (°C)	Current (A)
Benzoic acid Benzoic acid Benzoic acid Benzoic acid Benzoic acid Benzoic acid Benzoic acid Benzoic acid Benzoic acid Benzoic acid Benzoic acid Benzoic acid Benzoic acid Phenol Guaiacol	10 20 30 50 20 20 20 20 20 20 20 20 20 20 20	75 75 75 75 40 50 55 60 80 55 55 55 55 55 55	0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.05 0.1 0.2 0.6 0.1 0.1

For analysis, the samples were tested in the GC-MS Shimadzu TQ8040. The GC used a Restek Rtx-5 capillary column, 28.5 m × 0.25 mm with a 0.25 µm film thickness, a 1.18 ml min^− 1 helium carrier gas flow rate, and a split ratio of 1:25. The injector temperature was set at 250°C. The GC oven program started at 34°C and was held for 2 min and then heated at 50°C min^− 1 to 300°C. Mass spectrometry was operated at m/z ranging from 50 to 500. Species associated with each chromatographic peak were identified by comparing their observed mass spectrum with the NIST library. External standards were also used to identify compounds and quantify the peaks. Results are in area %.

Factorial design of experiment

The experimental matrix was designed using JMP PRO 17, as shown in Table 1, where a “+” signs indicate the compound is present, and a “-” signals the opposite. Each model compound and the initial concentration were selected as independent variables (factors), and the conversion rate and Faradaic efficiency as responses. The factors were set to discrete numeric values, and the model covered all main effects and two-factor interactions. Experiments with no model compounds (“-” “-” “-”) were not shown in Table 1, as no compounds would lead to 0% conversion. The initial concentration of each compound is always equal to the others. For all single-compound experiments, the duration was set to 4h, otherwise it was 5h. This adjustment was made to ensure enough electrons were introduced into the system, without a change in current.

Electrochemical measurements

Electrochemical measurements were performed using a PalmSens EmStat4s at 55°C, which consisted of a three-electrode system with the used cathode, anode, and an Ag/AgCl (in saturated KCl solution) as the working, counter, and reference electrodes, respectively. The potential vs. Ag/AgCl was converted to the potential vs.RHE (reversible hydrogen electrode) based on the Nernst equation:

$$\:{R}_{RHE}=\:{E}_{Ag/AgCl}+0.059\:x\:pH+{E}_{Ag/AgCl}^{0}$$

LSV tests were recorded at a scan rate of 5 mV s^− 1 in 1.0 M H₂SO₄ with and without the model compounds.

Calculations

To understand and quantify the ECH process, the following equations were used:

$$\:F.E.\:\left(\text{%}\right)=\frac{mol\:produced\:x\:n\:x\:F}{Total\:electrons\:passed}x\:100$$

$$\:Conversion\:\left(\text{%}\right)=\left(\frac{Moles\:reactant\:consumed}{Initial\:moles\:reactant}\right)x\:100$$

$$\:Selectivity\:\left(\text{%}\right)\:=\left(\frac{Moles\:desired\:product}{Total\:moles\:product}\right)x\:100$$

$$\:Current\:density\:\left(mA\:{cm}^{-2}\:\right)=\frac{Current}{Surface\:area}$$

$$\:Yeild\:\left(mMol\:{h}^{-1}\:\right)=\frac{Moles\:desired\:product}{Reaction\:duration}$$

Where F is the Faraday constant (96 485°C mol^− 1), and n is the number of electrons necessary for the hydrogenation process (n = 6 for the conversion of benzoic acid into cyclohexane carboxylic acid, for example).

DFT Calculations

All calculations were carried out using QuantumEspresso/7.2-foss-2022b ^29–31 with BURAI 1.3.1 as a visualiser. The generalized-gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional described the exchange-correlation energy. To enhance computational performance, all calculations were performed with a thermal smearing of 0.01 Ry and spin polarization. A 4-Layer 4 × 4 PtRu (111) slab was built with a 20 Å vacuum gap and two fixed bottom layers. The slab was placed at the bottom of the cell, to certify that all reactions happened on the top surface. A (2 × 2 × 1) k-point grid was set. The test molecule was placed at a distance of 1.5 Å from the surface, where it did not start the calculations with any bonds with the surface. The adsorption energy (Ead) of the different adsorbates is defined as:

$$\:{E}_{ad}=\:{E}_{Total}-({E}_{adsorbate}+{E}_{Surface})$$

Where E_Total is the calculated energy for the final system (slab + adsorbate), E_adsorbate is the calculated energy of each adsorbate and E_Surface is the energy of the bare cathode surface.

Data availability

The data supporting this study's findings are available from the corresponding authors upon reasonable request. Source data are provided in 10.6084/m9.figshare.26540506.

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Hansen, S., Mirkouei, A. & Diaz, L. A. A comprehensive state-of-technology review for upgrading bio-oil to renewable or blended hydrocarbon fuels. Renewable and Sustainable Energy Reviews 118, 109548 (2020).
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Grac̀a, I., Lopes, J. M., Cerqueira, H. S. & Ribeiro, M. F. Bio-oils Upgrading for Second Generation Biofuels. Ind Eng Chem Res 52, 275–287 (2012).
Hansen, S., Mirkouei, A. & Diaz, L. A. A comprehensive state-of-technology review for upgrading bio-oil to renewable or blended hydrocarbon fuels. Renewable and Sustainable Energy Reviews 118, 109548 (2020).
Lachos-Perez, D. et al. Review on Biomass Pyrolysis with a Focus on Bio-Oil Upgrading Techniques. Analytica 4, 182–205 (2023).
Grams, J., Jankowska, A. & Goscianska, J. Advances in design of heterogeneous catalysts for pyrolysis of lignocellulosic biomass and bio-oil upgrading. Microporous and Mesoporous Materials 362, 112761 (2023).
Chen, G. et al. Upgrading of Bio-Oil Model Compounds and Bio-Crude into Biofuel by Electrocatalysis: A Review. ChemSusChem 14, 1037–1052 (2021).
Guedes, R. E., Luna, A. S. & Torres, A. R. Operating parameters for bio-oil production in biomass pyrolysis: A review. J Anal Appl Pyrolysis 129, 134–149 (2018).
Li, Z. et al. Green Chemistry Mild electrocatalytic hydrogenation and hydrodeoxygenation of bio-oil derived phenolic compounds using ruthenium supported on activated carbon cloth. (2012) doi:10.1039/c2gc35552c.
Dyer, A. C., Nahil, M. A. & Williams, P. T. Catalytic co-pyrolysis of biomass and waste plastics as a route to upgraded bio-oil. Journal of the Energy Institute 97, 27–36 (2021).
Toraman, H. E., Dijkmans, T., Djokic, M. R., Van Geem, K. M. & Marin, G. B. Detailed compositional characterization of plastic waste pyrolysis oil by comprehensive two-dimensional gas-chromatography coupled to multiple detectors. J Chromatogr A 1359, 237–246 (2014).
Čepelioʇullar, Ö. & Pütün, A. E. Products characterization study of a slow pyrolysis of biomass-plastic mixtures in a fixed-bed reactor. J Anal Appl Pyrolysis 110, 363–374 (2014).
Zhou, Y. et al. Electrocatalytic Upgrading of Lignin-Derived Bio-Oil Based on Surface-Engineered PtNiB Nanostructure. Adv Funct Mater 29, 1807651 (2019).
Lam, C. H. et al. Electrocatalytic upgrading of model lignin monomers with earth abundant metal electrodes. 17, 601 (2015).
Garedew, M., Young-Farhat, D., Jackson, J. E. & Saffron, C. M. Electrocatalytic Upgrading of Phenolic Compounds Observed after Lignin Pyrolysis. ACS Sustain Chem Eng 7, 8375–8386 (2019).
Du, Y., Chen, X. & Liang, C. Selective electrocatalytic hydrogenation of phenols over ternary Pt3RuSn alloy. Molecular Catalysis 535, (2023).
Du, Y. et al. Electrocatalysis as an efficient alternative to thermal catalysis over PtRu bimetallic catalysts for hydrogenation of benzoic acid derivatives †. (2023) doi:10.1039/d3gc01540h.
Fukazawa, A., Shimizu, Y., Shida, N. & Atobe, M. Organic & Biomolecular Chemistry Electrocatalytic hydrogenation of benzoic acids in a proton-exchange membrane reactor †. Org. Biomol. Chem 19, 7363–7368 (2021).
Salakhum, S., Yutthalekha, T., Shetsiri, S., Witoon, T. & Wattanakit, C. Bifunctional and Bimetallic Pt–Ru/HZSM-5 Nanoparticles for the Mild Hydrodeoxygenation of Lignin-Derived 4-Propylphenol. ACS Appl Nano Mater 2, 1053–1062 (2019).
Roessler, A., Dossenbach, O. & Rys, P. Electrocatalytic Hydrogenation of Indigo. J Electrochem Soc 150, D1 (2003).
Sanyal, U., Koh, K., Meyer, L. C., Karkamkar, A. & Gutiérrez, O. Y. Simultaneous electrocatalytic hydrogenation of aldehydes and phenol over carbon-supported metals. J Appl Electrochem 51, 27–36 (2021).
Liang, L. et al. Interactions of phenol and benzaldehyde in electrocatalytic upgrading process. Chinese Chemical Letters 35, 108581 (2024).
Sanyal, U. et al. Hydrogen Bonding Enhances the Electrochemical Hydrogenation of Benzaldehyde in the Aqueous Phase. Angewandte Chemie International Edition 60, 290–296 (2021).
Peng, J. et al. Effect of Boiling Pretreatment on Physicochemical and Transport Properties of Perfluorosulfonic Acid Membrane. ACS Appl Polym Mater 5, 9940–9951 (2023).
Giannozzi, P. et al. Quantum ESPRESSO toward the exascale. J Chem Phys 152, (2020).
Giannozzi, P. et al. Advanced capabilities for materials modelling with Quantum ESPRESSO. Journal of Physics: Condensed Matter 29, 465901 (2017).
Giannozzi, P. et al. QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials. Journal of Physics: Condensed Matter 21, 395502 (2009).

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Chawla, S., Sajan Varghese, B., Ghazanfar Hussain, C., Keçili, R. & Mustansar Hussain, C. Environmental impacts of post-consumer plastic wastes: Treatment technologies towards eco-sustainability and circular economy. (2022) doi:10.1016/j.chemosphere.2022.135867.
Zhang, Q., Chang, J., Wang, T. & Xu, Y. Review of biomass pyrolysis oil properties and upgrading research. Energy Convers Manag 48, 87–92 (2007).
Hansen, S., Mirkouei, A. & Diaz, L. A. A comprehensive state-of-technology review for upgrading bio-oil to renewable or blended hydrocarbon fuels. Renewable and Sustainable Energy Reviews 118, 109548 (2020).
Kumar, R. & Strezov, V. Thermochemical production of bio-oil: A review of downstream processing technologies for bio-oil upgrading, production of hydrogen and high value-added products. Renewable and Sustainable Energy Reviews 135, 110152 (2021).
Grac̀a, I., Lopes, J. M., Cerqueira, H. S. & Ribeiro, M. F. Bio-oils Upgrading for Second Generation Biofuels. Ind Eng Chem Res 52, 275–287 (2012).
Hansen, S., Mirkouei, A. & Diaz, L. A. A comprehensive state-of-technology review for upgrading bio-oil to renewable or blended hydrocarbon fuels. Renewable and Sustainable Energy Reviews 118, 109548 (2020).
Lachos-Perez, D. et al. Review on Biomass Pyrolysis with a Focus on Bio-Oil Upgrading Techniques. Analytica 4, 182–205 (2023).
Grams, J., Jankowska, A. & Goscianska, J. Advances in design of heterogeneous catalysts for pyrolysis of lignocellulosic biomass and bio-oil upgrading. Microporous and Mesoporous Materials 362, 112761 (2023).
Chen, G. et al. Upgrading of Bio-Oil Model Compounds and Bio-Crude into Biofuel by Electrocatalysis: A Review. ChemSusChem 14, 1037–1052 (2021).
Guedes, R. E., Luna, A. S. & Torres, A. R. Operating parameters for bio-oil production in biomass pyrolysis: A review. J Anal Appl Pyrolysis 129, 134–149 (2018).
Li, Z. et al. Green Chemistry Mild electrocatalytic hydrogenation and hydrodeoxygenation of bio-oil derived phenolic compounds using ruthenium supported on activated carbon cloth. (2012) doi:10.1039/c2gc35552c.
Dyer, A. C., Nahil, M. A. & Williams, P. T. Catalytic co-pyrolysis of biomass and waste plastics as a route to upgraded bio-oil. Journal of the Energy Institute 97, 27–36 (2021).
Toraman, H. E., Dijkmans, T., Djokic, M. R., Van Geem, K. M. & Marin, G. B. Detailed compositional characterization of plastic waste pyrolysis oil by comprehensive two-dimensional gas-chromatography coupled to multiple detectors. J Chromatogr A 1359, 237–246 (2014).
Čepelioʇullar, Ö. & Pütün, A. E. Products characterization study of a slow pyrolysis of biomass-plastic mixtures in a fixed-bed reactor. J Anal Appl Pyrolysis 110, 363–374 (2014).
Zhou, Y. et al. Electrocatalytic Upgrading of Lignin-Derived Bio-Oil Based on Surface-Engineered PtNiB Nanostructure. Adv Funct Mater 29, 1807651 (2019).
Lam, C. H. et al. Electrocatalytic upgrading of model lignin monomers with earth abundant metal electrodes. 17, 601 (2015).
Garedew, M., Young-Farhat, D., Jackson, J. E. & Saffron, C. M. Electrocatalytic Upgrading of Phenolic Compounds Observed after Lignin Pyrolysis. ACS Sustain Chem Eng 7, 8375–8386 (2019).
Du, Y., Chen, X. & Liang, C. Selective electrocatalytic hydrogenation of phenols over ternary Pt3RuSn alloy. Molecular Catalysis 535, (2023).
Du, Y. et al. Electrocatalysis as an efficient alternative to thermal catalysis over PtRu bimetallic catalysts for hydrogenation of benzoic acid derivatives †. (2023) doi:10.1039/d3gc01540h.
Fukazawa, A., Shimizu, Y., Shida, N. & Atobe, M. Organic & Biomolecular Chemistry Electrocatalytic hydrogenation of benzoic acids in a proton-exchange membrane reactor †. Org. Biomol. Chem 19, 7363–7368 (2021).
Salakhum, S., Yutthalekha, T., Shetsiri, S., Witoon, T. & Wattanakit, C. Bifunctional and Bimetallic Pt–Ru/HZSM-5 Nanoparticles for the Mild Hydrodeoxygenation of Lignin-Derived 4-Propylphenol. ACS Appl Nano Mater 2, 1053–1062 (2019).
Roessler, A., Dossenbach, O. & Rys, P. Electrocatalytic Hydrogenation of Indigo. J Electrochem Soc 150, D1 (2003).
Sanyal, U., Koh, K., Meyer, L. C., Karkamkar, A. & Gutiérrez, O. Y. Simultaneous electrocatalytic hydrogenation of aldehydes and phenol over carbon-supported metals. J Appl Electrochem 51, 27–36 (2021).
Liang, L. et al. Interactions of phenol and benzaldehyde in electrocatalytic upgrading process. Chinese Chemical Letters 35, 108581 (2024).
Sanyal, U. et al. Hydrogen Bonding Enhances the Electrochemical Hydrogenation of Benzaldehyde in the Aqueous Phase. Angewandte Chemie International Edition 60, 290–296 (2021).
Peng, J. et al. Effect of Boiling Pretreatment on Physicochemical and Transport Properties of Perfluorosulfonic Acid Membrane. ACS Appl Polym Mater 5, 9940–9951 (2023).
Giannozzi, P. et al. Quantum ESPRESSO toward the exascale. J Chem Phys 152, (2020).
Giannozzi, P. et al. Advanced capabilities for materials modelling with Quantum ESPRESSO. Journal of Physics: Condensed Matter 29, 465901 (2017).
Giannozzi, P. et al. QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials. Journal of Physics: Condensed Matter 21, 395502 (2009).

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Insights into the Electrochemical Hydrogenation Reaction Mechanisms of Aromatic Compounds Mixtures over Bimetallic PtRu Catalyst

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Abstract

Figures

Introduction

Results

Electrochemical hydronation of model compounds

Electrochemical hydrogenation of model compound mixtures

Factorial design analysis

Density Functional Theory (DFT) calculations

Material characterisation

Discussion

Methods

Materials

ECH of pure and mixed model compounds

Factorial design of experiment

Electrochemical measurements

Calculations

DFT Calculations

Declarations

Data availability

References

Additional Declarations

Supplementary Files

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